[@Xavier Bloodbayne]

Searching for white knights on google, this image is inspiring: 
[hider=Image][img]https://lh3.googleusercontent.com/-Gb8WNenjBDA/VxazxZwjmaI/AAAAAAAAAII/x65tPO9B-6saGanevzxr_Dly_Nfa43iiQ/w506-h750/2016-04-16.png[/img][/hider]

For the battle to work she would have to be at least as powerful as he would be by the end of the fight for the duration of the fight. That means if she allows it to last that long for him to be powering up she must either be cocky or merciful unless you have another idea. I could go with cocky and loves to fight, even if you might not picture a white knight as such, I still think it's possible. Perhaps sort of like Vegeta, wanting to test herself against an equal opponent. Of course this would backfire if he goes on to attack the village, having killed her for a large power boost more than likely. 

Like she didn't have time to release her full power because he takes her off guard with a underhanded attack.However, when she comes back to life later on she can try to make up for her mistake and fight him at her full power against his full powered form.